The present invention relates to treating fluids comprising sulfonated gelling agent polymers, and methods of use in treating subterranean formations.
Viscous treating fluids are used in a variety of operations and treatments in oil and gas wells. Such operations and treatments include forming gravel packs in well bores, fracturing producing zones, performing permeability control treatments and the like. As used herein, the term “treatment,” or “treating,” refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The terms “treatment,” and “treating,” as used herein, do not imply any particular action by the fluid or any particular component thereof.
Hydrocarbon producing wells are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing, a viscous fracturing fluid, which also functions as a carrier fluid, is pumped into a subterranean formation to be fractured at a rate and pressure such that one or more fractures are formed or enhanced in the formation. “Enhancing” one or more fractures in a subterranean formation, as that term is used herein, is defined to include the extension or enlargement of one or more natural or previously created fractures in the subterranean formation. Proppant particles, e.g., graded sand, for propping the fractures open are suspended in the fracturing fluid and are deposited in the fractures when the fracturing fluid viscosity is reduced. The fracturing fluid viscosity is reduced by including a delayed viscosity breaker in the fracturing fluid that causes it to revert to a thin fluid. The proppant particles deposited in the fractures function to prevent the fractures from closing so that conductive channels are formed through which produced hydrocarbons can readily flow.
Aqueous treating fluids are generally viscosified by mixing a hydratable polysaccharide gelling agent polymer with water. For example, guar gum and its derivatives are often used to viscosity aqueous fracturing fluids. Guar gum is a random coil polymer that can be readily crosslinked with various cross-linking agents, e.g., metal ions. Once crosslinked, guar and guar derivatives can form highly viscoelastic gels that approach near zero suspended particle settling rates.
Often increasing the effectiveness of gelling agent polymers is desirable, and this has been achieved to a significant degree by grafting ionic groups, for example carboxyl groups, onto the gelling agent polymer chain. Since like charges tend to repel each other, the carboxyl groups are thought to force the flexible coiled polymer to become more linear. Maximizing the linearity is thought to result in an enlarged radius of gyration, which in turn is thought to result in a lesser amount of gelling agent being required to generate a gelled treating fluid.
One problem with using gelling agent polymers comprising ionic groups is that the viscosity-increasing effect of carboxyl groups is thought to be greatly reduced as the pH of the fluid drops below 7. This is thought to be due to the fact that the carboxylate ions are salts of weak acids and tend to hydrolyze. In addition, the solubility of anionic groups in water containing multivalent metal ions such as calcium and magnesium is low, making gelling agent polymers containing anionic groups, e.g., carboxyl groups, generally less soluble in hard water. Thus, anionic groups is thought to render the viscosity of a treating fluid that comprises conventional gelling agent polymers sensitive to ionic strength, whereby the viscosity of the treating fluid that comprises one or more water soluble salts is thought to be much less than the viscosity in fresh water. This sensitivity to ionic strength may be undesirable since the aqueous liquids used in well treating fluids often contain salts, for example, when the treating fluid comprises saltwater, brines, seawater, produced water (e.g., naturally-occurring water found in a subterranean formation), or flowback water (e.g., water that was previously placed in a subterranean formation, for example, in the course of performing another operation), when the subterranean formation comprises salts that dissolve in the treating fluid, and/or as an additive to inhibit swelling of formation clays.